This invention relates to a protective relaying system of an electric power transmission line capable of operating at a high reliability by transmitting and processing data having a relatively long code length, more particularly a protective relaying system wherein electric quantities at respective terminals of a transmission line are transmitted as encoded digital signals and arithmetically operated so as to protect the transmission line.
FIG. 1 is a block diagram of a protective relaying system to which the invention is applicable in which electric stations A and B are interconnected by a transmission line TL including circuit breakers CB.sub.A and CB.sub.B and current transformers CT.sub.A and CT.sub.B. The relaying system comprises relay units RY.sub.A and RY.sub.B, analogue-digital converters (A/D converter) AD.sub.A and AB.sub.B respectively connected between current transformers CT.sub.A, CT.sub.B and relay units RY.sub.A, RY.sub.B, transmitters SD.sub.A and SD.sub.B respectively connected to the outputs of A/D converters and receivers RC.sub.A and RC.sub.B respectively connected to relay units RY.sub.A and RY.sub.B.
The secondary current I.sub.A of current transformer CT.sub.A at station A is converted into a digital quantity i.sub.AA by A/D converter AD.sub.A. A portion of the digital quantity i.sub.AA is supplied to the relay unit RY.sub.A of station A whereas the other portion is modulated, for example by PCM (pulse code modulation), in the transmitter SD.sub.A and then sent to the other station B as a modulated signal S.sub.A. In station B, this signal is demodulated by a receiver RC.sub.B to produce a signal i.sub.AB which is supplied to relay unit RY.sub.B together with a digital signal i.sub.BB obtained by converting the secondary current I.sub.B of current transformer CT.sub.B by D/A converter AD.sub.B, thus effecting differential protection of the transmission line with digital quantities.
Suppose now that current normally flows from station A to station B. Then, so long as the vector sum of currents I.sub.A and I.sub.B is zero the differential relaying system would not operate. However, when the vector sum exceeds a predetermined value, the relay units operates to trip both circuit breakers CB.sub.A and CB.sub.B. With this construction, when the transmitted signal S.sub.A is affected by such disturbance of the transmission line as noise, the demodulated output i.sub.AB would no more correspond to current I.sub.A, thus causing the relay to misoperate. To overcome this difficulty, it is usual to prevent such misoperation of the relay unit by detecting an abnormal condition of the transmitted signal.
There are the following methods of detecting the abnormal condition:
1. To make long the transmitted code signal by adding a parity code thereto so as to detect code errors. PA0 2. Code errors are detected or corrected by using such long code as a hamming code. PA0 3. To repeat the digital arithmetic operation at the relay unit for producing an operation instruction when a definite operating condition is continuously satisfied for more than a predetermined times, for example 3.
Under normal operating conditions, a single or "one shot" disturbance occurs randomly on a transmission line, but when a radio repeating office is constructed or repaired or when fading occurs due to unstable atmospheric conditions, the condition of the transmission line would become worse in a short time. However, such worse condition occurs scarecely.
In a protective relaying system utilizing a digital arithmetic operation, judgment of the fault is usually made by using a number of time series signals i.sub.AB (i.sub.AB (t), i.sub.AB (t+.DELTA.t) . . . ,) where t represents time and .DELTA.t a time step, the adverse effect upon the transmitted signal persists a long time. To overcome this difficulty, when the condition of the transmission line becomes worse it has been the practice to increase the lock time of the relay when a code error is detected by the above described method 1 or 2 or to increase the number of repetitions in method 3, for the purpose of preventing misoperation of the relay. Each of these measures prevents high speed operation of the relay under favorable conditions of the transmission line. When the condition of the transmission line becomes an extremely worst condition and code errors occurs in a burst it has been difficult to avoid the effect of such adverse condition even with a long lock time.